The invention relates to a method for determining the filling mass of a cryogenically stored gas in a container, which contains a medium that is in one part in the liquid phase and in the other part in the gaseous phase, and also to a device for carrying out the method and to a use thereof in a motor vehicle.
Various measuring methods are used for determining the filling mass of a cryogenically stored gas in a container for indicating, checking and controlling purposes, but they are to some extent greatly restricted with regard to their area of use and their accuracy. Along with other measuring methods, one for example is based on the determination of the pressure difference between the bottom region and the top region of a container. However, the accuracy of this method is often insufficient, since for example the measured values are strongly temperature-dependent. In containers in which ambient pressure prevails, there is the possibility of determining the filling level by a temperature sensor inside the container, since the liquid fraction of a medium that is partly in the liquid phase and partly in the gaseous phase and is under ambient pressure is always colder than the gaseous phase. In the case of containers containing boiling liquid, the phase state at the measuring point is detected on the basis of whether this temperature sensor at the measuring point indicates a temperature greater than or less than the boiling temperature of the medium. This method can be used in a container that is filled with boiling liquid and is under a pressure greater than ambient pressure, since the boiling temperature changes with the pressure and fluctuations in the filling level are also accompanied by fluctuations in pressure. Furthermore, in a container filled as described, a state of equilibrium between the liquid phase and the gaseous phase is established over time. In the case of containers in which the content is under a pressure higher than ambient pressure, on the other hand, capacitive measuring methods may be used. If, however, the container is under high pressure and at low temperatures, this method encounters difficulties, since the sensors available are not suitable for low temperatures and cannot be used in this area. In addition, in the case of capacitive measurements, great errors occur if the cold sensor enters the gas space and condensation takes place on the surface of the sensor.
As a result of heat entering an insulated cryogenic tank system, a thermally induced stratification of the stored medium, for example hydrogen for driving a motor vehicle, occurs, with warm gas collecting in the upper region and cold gas collecting in the lower region of the container. For determining the filling level of such a tank or container, it is also possible to determine the temperature of the medium that is representative for the calculation of the density. This is only possible by measuring the temperature of the medium at multiple representative measuring points inside the tank or outside the tank.
Thus, German Utility Model 296 15 453 describes, for a filling level indication, arranging a sensor chain comprising resistance thermometers and a reference sensor in a container. The reference sensor is in the liquid and it is ascertained by means of the sensor chain which sensor is no longer in the liquid, in turn allowing the level of the liquid of the filling to be deduced.
For this purpose, multiple temperature sensors are required, which increases the cost, complexity and susceptibility to errors. The position of the measuring points must be laboriously determined and ensured throughout the service life. It is also very laborious to exchange defective sensors in the container.
The invention is based on the object of providing a method and a device for determining the filling mass of a cryogenically stored gas in a container which, while involving a simple construction, deliver sufficiently accurate results and avoid the aforementioned disadvantages. Advantageous forms and developments are the content of the dependent claims.
The invention provides a method for determining a filling mass in a thermally-insulated container for a cryogenically stored gas, wherein the filling mass is determined by way of a known container volume and a calculated density of the gas content of the container, the value of which is calculated from a container pressure measurement and a temperature measurement. The method is characterized in that a temperature sensor is used for measuring a mixing temperature of a liquid phase and a gaseous phase of the gas, wherein the liquid phase is extracted by way of a first extraction line at the geodetically lowest point and the gaseous phase is extracted by way of a second extraction line at the geodetically highest point of the container cavity. Moreover, the temperature sensor is placed downstream of the extraction points, after a convergence of the first and second extraction lines to a single extraction line leading out of the container, on the inside or outside of said line, at a location where complete and thorough mixing of the liquid phase and the gaseous phase of the gas from the first and second extraction lines has already taken place.
This method is advantageously carried out with a device for determining the filling mass in a thermally insulated container which contains a medium that is in one part in the liquid phase and in another part in the gaseous phase, which is characterized in that the container is equipped with a pressure gage and with a temperature sensor, which measures a mixing temperature of the liquid phase and the gaseous phase of the gas, for which purpose the liquid phase is extracted by way of a first extraction line at the geodetically lowest point and the gaseous phase is extracted by way of a second extraction line at the geodetically highest point of the container cavity. Moreover, the temperature sensor is placed downstream of the extraction points, after a convergence of the first and second extraction lines to a single extraction line leading out of the container, on the inside or outside of said line, at a location where complete and thorough mixing of the liquid phase and the gaseous phase of the gas from the first and second extraction lines has already taken place.
This has the advantage that the temperature sensor, which is positioned downstream of the extraction points in the extraction line convergence, measures the mixing temperature, and consequently the averaged temperature of the medium that is representative for the filling level determination. Assuming a linear temperature stratification over the height of the container, extracting the medium with the maximum temperature and the minimum temperature makes a very accurate determination of the temperature of the medium possible, since it is ensured that both the coldest gas and the warmest gas occurring in the tank are extracted in equal parts. Only one sensor is required, and it can additionally be arranged in such a way that it can be easily changed in the event of damage. Using a simple series of measurements, it can be calibrated in such a way that possible influences resulting from its positioning along the extraction line can be eliminated.
Therefore, a preferred method is characterized in that stored calibration or correction values or calibration or correction functions are used for making allowance at least for the position of the temperature sensor in the determination of the filling mass.
A further preferred method provides that the filling mass of the container is determined as and when desired or is automatically determined repeatedly at time intervals.
This is advantageous for regularly renewing the indication if a container is used as a store for receiving the cryogenic gas as an operating medium of a drive unit of a motor vehicle and is held in a thermally insulated manner in an outer container that is accommodated in the motor vehicle and if the motor vehicle has in its interior an indicating device that presents to a driver of the motor vehicle the last-calculated filling mass, at least in comparison with a maximum filling mass and a minimum filling mass.
Advantageous devices for carrying out the method are distinguished by the fact that the connecting point of the two extraction lines or the single extraction line is configured in such a way that complete and thorough mixing of the liquid phase and the gaseous phase of the gas from the first and second extraction lines takes place. If the temperature sensor is located outside the container, even easier maintenance is possible.
It is advantageous for the calculation of the filling mass if the device has a control device with a computing device, which determines the filling mass of the container as and when desired or automatically repeatedly at time intervals. In this case, calibration or correction values or calibration or correction functions stored in the computing device may be used for making allowance at least for the position of the temperature sensor in the determination of the filling mass.
The following description with the associated drawing describes a preferred exemplary embodiment of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.